Make cache-manager tests strictNullChecks safe (#42686)
Make cache-manager tests strictNullChecks safe (#42686)

The new code in cache-manager wasn't strictNullChecks safe, which caused
problems because a community member enabled strictNullChecks in most
packages during the review period for that code.

This change just fixes the tests, although the optional methods on Store
are a bit suspicious. From skimming cache-manager's documentation, I
couldn't figure out if having the methods be optional was accurate.
dsebastien/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped